This invention relates in general to curable compositions that cure into two phases or two polymer networks and that are suitable as encapsulant compositions. They have particular use in semiconductor packaging applications. This invention further relates to encapsulant compositions used in an assembly of a semiconductor die or semiconductor package (either hereinafter semiconductor) attached to a substrate in which a gap created between the semiconductor and the substrate is filled with the encapsulant composition. The first of the two separate phases or networks has a high modulus and the second has a low modulus phase.
In the manufacture of semiconductors, electrical connections are made between electrical terminals on the semiconductor and corresponding electrical terminals on the substrate for the semiconductor. One method for making these interconnections uses a metallic or polymeric solder applied to the terminals. The terminals are aligned and contacted together and the resulting assembly of semiconductor and substrate is heated to reflow the solder and solidify the connection. The space between the semiconductor and substrate created by the solder interconnections is filled with a polymeric encapsulant (underfill), which after deposition is cured. The cured encapsulant acts to reinforce the interconnect and to absorb stress associated with temperature cycling during further fabrication and ultimate operation of the device containing the semiconductor. The stress results from the disparate coefficients of thermal expansion (CTE) of the semiconductor and the substrate.
To be reliable for reinforcing the solder connections, the underfill encapsulant should have a high modulus value; the higher the modulus, the harder it is to deform that material and the higher the support of the solder interconnects. The encapsulant should also have a high glass transition temperature, Tg, when cured, at least as high as, or sufficiently close to, the ceiling temperature in any temperature cycling operation, so that it remains intact in order to protect the solder joints.
During the subsequent cooling of the semiconductor after reflow of the solder, warpage or deformation of the semiconductor may occur. A warped semiconductor is more difficult to attach to the next substrate. As die size and die number increase per semiconductor package, the problem becomes compounded. The use of Pb free solders also contributes to warpage because Pb free solders reflow at higher temperatures than Pb solders, and higher temperatures generate more warpage. A low modulus material would help to minimize warpage, as a low modulus material deforms easily and can absorb stress, but this is contrary to the requirement of high modulus for reliability of the underfill encapsulant.
The current solution to this problem is the addition of soft particles, such as core shell rubber to toughen the underfill material, or hard particles, such as silica, to lower the coefficient of thermal expansion of the underfill. However, using filler to manipulate the physical properties of underfill often involves a viscosity penalty and makes the composition unsuitable.